CSE3840 - How the conditions are to be interpreted: Directly Necessary services: Can the 85% test be applied on a sector/account basis? (guidance effective until 14 August 2018)
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
HMRC will, in certain circumstances, accept the 85 per cent test being applied to particular parts of a member’s business provided the test can be reasonably applied. For example if the member has a sectorised partial exemption and business/non-business method they can apply the test on a sector basis. If a particular sector qualifies, the member can receive all supplies from their CSG that are attributable to that sector on an exempt basis. The test could also be applied on the same basis if, for example, a member can identify specific account codes in its accounting system; if the test is satisfied on that basis a member can receive all supplies from their CSG that are directly attributable to that account code on an exempt basis.
In these circumstances the test would have to be applied to external supplies and not to supplies made and received within the same VAT entity as they are neither exempt nor non-business supplies for the purpose of the Cost Sharing Exemption.